Grazing and normal incidence interferometer having common reference surface

ABSTRACT

A system for inspecting specimens such as semiconductor wafers is provided. The system provides scanning of dual-sided specimens using a diffraction grating that widens and passes nth order (n&gt;0) wave fronts to the specimen surface and a reflective surface for each channel of the light beam. Two channels and two reflective surfaces are preferably employed, and the wavefronts are combined using a second diffraction grating and passed to a camera system having a desired aspect ratio. The system preferably comprises a damping arrangement which filters unwanted acoustic and seismic vibration, including an optics arrangement which scans a first portion of the specimen and a translation or rotation arrangement for translating or rotating the specimen to a position where the optics arrangement can scan the remaining portion(s) of the specimen. The system further includes-means for stitching scans together, providing for smaller and less expensive optical elements.

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/157,341, entitled “Reduced Coherence Symmetric GrazingIncidence Differential Interferometer,” filed Jun. 20, 2005, which is acontinuation of U.S. patent application Ser. No. 09/543,604, entitled“Reduced Coherence Symmetric Grazing Incidence DifferentialInterferometer,” filed Apr. 7, 2000, now U.S. Pat. No. 7,057,741, whichis a continuation in part of U.S. patent application Ser. No.09/335,673, entitled “Method and Apparatus for Scanning, Stitching andDamping Measurements of a Double-Sided Inspection Tool,” filed on Jun.18, 1999, now U.S. Pat. No. 6,414,752, all of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of optical imagingand more particularly to systems for sub-aperture data imaging of doublesided interferometric specimens, such as semiconductor wafers.

2. Description of the Related Art

The progress of the semiconductor industry over the last years hasresulted in a sharp increase in the diameters of semiconductor wafers asbase material for chip production for economic and process technicalreasons. Wafers having diameters of 200 and 300 millimeters arecurrently processed as a matter of course.

At present manufacturers and processors of wafers in the 200 and 300 mmrange do not have a wide range of measuring devices available whichenable inspection of particular geometric features, namely flatness,curvature, and thickness variation, with sufficient resolution andprecision.

As scanning of specimens has improved to the sub-aperture range, thetime required to perform full specimen inspection for a dual-sidedspecimen has also increased. Various inspection approaches have beenemployed, such as performing an inspection of one side of the specimen,inverting the specimen, and then inspecting the other side thereof. Sucha system requires mechanically handling the specimen, which isundesirable. Further, the act of inspecting the specimen has generallyrequired binding the specimen, which can cause deformation at the edgesof the specimen, increase defects at the edge, or cause bending of thespecimen during inspection.

One method for inspecting both sides of a dual sided specimen isdisclosed in PCT Application PCT/EP/03881 to Dieter Mueller andcurrently assigned to the KLA-Tencor Corporation, the assignee of thecurrent application. The system disclosed therein uses a phase shiftinginterferometric design which facilitates the simultaneous topographymeasurement of both sides of a specimen, such as a semiconductor wafer,as well as the thickness variation of the wafer. A simplified drawing ofthe Mueller grazing incidence interferometer design is illustrated inFIG. 1A. The system of FIG. 1A uses a collimated laser light source 101along with a lens arrangement 102 to cause grazing of light energy offthe surface of both sides of the specimen 103 simultaneously. A secondlens arrangement 104 then provides focusing of the resultant lightenergy and a detector 105 provides for detection of the light energy.

The design of FIG. 1A is highly useful in performing topographicalmeasurements for both sides of a dual-sided specimen in a singlemeasurement cycle, but suffers from some drawbacks. First, the systemrequires minimum specimen movement during measurement, which can bedifficult due to vibration in the surrounding area and vibration of thespecimen itself. Further, the inspection can be time consuming andrequires highly precise light energy application and lensing, which isexpensive. The specimen must be free standing and free of edge forces,and the incidence geometry during inspection must be unimpeded.Illumination access must be preserved under all incidence angles. Thesefactors provide mechanical challenges for successfully supporting thespecimen; excessive application of force at a minimum number of pointsmay deform the specimen, while numerous contact points impede access andrequire exact positioning to avoid specimen deformation or bendingduring inspection. Further, edge support of the specimen has a tendencyto cause the specimen to act like a membrane and induce vibration due toslight acoustic or seismic disturbances. This membrane tendency combinedwith the other problems noted above have generally been addressed byincluding most components of the system within an enclosure thatminimizes ambient vibrations, which adds significant cost to the systemand does not fully solve all vibration problems.

Further, the previous system has a tendency to require excessivecoherence lengths. As is generally known in the art, the coherencelength is the distance along the emitted laser beam over which the laserlight has sufficient coherence to produce visible interference fringes.Coherence length is important when a laser beam is split and recombinedto form an interference pattern, as in the system presented in FIG. 1A.

In general, when a laser beam is split, the optical path difference isthe difference in length between the two paths before recombining. Ifthe optical path difference is less than the longitudinal spatialcoherence length of the light beam, interference fringes are formed atthe receiving element, or screen. If the optical path difference isgreater than the longitudinal spatial coherence length, no interferencefringes form. Thus it is desirable to have a small spatial coherencelength to minimize the size of the components involved.

The system of FIG. 1A provides a high spatial coherence between thereference wave fronts and the specimen wave fronts. Such a system makesthe overall measurement system highly sensitive to background noisealong the optical path. The noise creates a diffraction pattern on topof the measurement signal and thus degrades the image obtained of thesurfaces. In particular, the background signal tends to be unstable andcan be difficult to correct using compensation techniques.

The cost of lenses sized to accommodate inspection of a full wafer inthe arrangement shown in FIG. 1A is significant, and such lensesgenerally have the same diameter as the diameter of the specimen, on theorder of 200 or 300 millimeters depending on the application. Fullaperture decollimating optics, including precision lenses, gratings, andbeamsplitters used in a configuration for performing full inspection ofa 300 millimeter specimen are extremely expensive, generally costingorders of magnitude more than optical components half the diameter ofthe wafer.

Further, the system disclosed in FIG. 1A requires a high spatialcoherence between the reference wave fronts and the specimen wavefronts, making the system sensitive to background noise along theoptical path. Noise creates a diffraction pattern that increases themeasurement signal in a random fashion. The result unstable andcompensation for the combined effect is extremely difficult.

It is an object of the current system to provide a system having arelatively small spatial coherence length to minimize system sensitivityto background noise along the optical path and permit use of reasonablysized enclosure components.

It is another object of the current invention to provide a system forperforming a single measurement cycle inspection of a dual-sidedspecimen having dimensions up to and greater than 300 millimeters.

It is a further object of the present invention to provide a system forinspection of dual-sided specimens without requiring an excessive numberof binding points and simultaneously allowing free access for inspectionof both sides of the specimen.

It is a further object of the current invention to provide for thesingle measurement cycle inspection of a dual-sided specimen whileminimizing the tendency for the specimen to behave as a membrane andminimize any acoustic and/or seismic vibrations associated with theinspection apparatus and process.

It is still a further object of the present invention to accomplish allof the aforesaid objectives at a relatively low cost, particularly inconnection with the collimating and decollimating optics and anyenclosures required to minimize acoustic and seismic vibrations.

SUMMARY OF THE INVENTION

The present invention is a system for inspecting a wafer, includinginspecting both sides of a dual sided wafer or specimen. The wafer ismounted using a fixed three point mounting arrangement that holds thewafer at a relatively fixed position while simultaneously minimizingbending and stress. Light energy is transmitted through a lensarrangement employing lenses having diameter smaller than the specimen,such as half the size of the specimen, arranged to cause light energy tostrike the surface of the wafer and subsequently pass through secondcollimating lens where detection and observation is performed.

The inventive system includes a variable coherence light source thattransmits light energy through a collimator, which splits the lightenergy into two channels and directs said light energy to a diffractiongrating. The diffraction grating splits each of the two beams into twoseparate first order beams, or a total of four first order beams. Two ofthese first order beams are directed to the wafer surface, while theother two are directed toward flat reflective surfaces facing the wafersurfaces. Another diffraction grating is positioned to receive the fourfirst order beams and combine said beams into two separate channels,each of which are directed to a separate camera. Each camera isspecially designed to receive the signal provided and resolve the imageof the wafer surface.

In an alternate arrangement, the system includes at least one lightsource mounted proximate and substantially parallel to a flat in thearrangement previously described. The purpose of this optional source isto provide a catadioptric inspection of the surface. The light source,such as a helium-neon laser, passes through a beamsplitter, through acollimator, through the flat and strikes the wafer surface. The lightbeam then reflects off the wafer surface, passes through the flat,through the collimator, is deflected by the beamsplitter, and isreceived by a camera element or other sensing device.

The system optionally employs a calibration object for distortioncalibration needed to match the front side and back side images of thewafer to determine the thickness variation of the wafer.

The system preferably includes at least one damping bar, where thenumber of damping bars depends on the wafer repositioning arrangement.The effect of the damping bar is to perform viscous film damping, orVFD, of the non-measured surface of the specimen to minimize the effectsof vibration in accordance with VFD, or the Bernoulli principle. Eachdamping bar is positioned to be within close proximity of the surface tobe damped. The proximity between any damping bar and the surface of thewafer is preferably less than 0.5 millimeters, and spacing of 0.25 and0.33 may be successfully employed. Smaller gaps provide problems whenwarped specimens are inspected. One embodiment of the current inventionemploys a damping bar to cover slightly less than half of the specimenwhen in scanning position.

Mounting for the wafer uses a three point kinematic mount. The mountingpoints include clips having spherical or semi-spherical tangentiallymounted contacts, mounted to a support plate and arranged to besubstantially coplanar, where the clips are adjustable to provide forslight irregularities in the shape of the wafer. The adjustability ofthe contact points provide the ability to hold the wafer without a stiffor hard connection, which could cause bending or deformation, as well aswithout a loose or insecure connection, which could cause inaccuratemeasurements.

A wafer or specimen to be measured is held on a holding device such thatboth plane surfaces are arranged in vertical direction parallel to thelight beam P. The wafer is supported substantially at its vertical edgeso that both surfaces are not substantially contacted by the supportpost and are freely accessible to the interferometric measurement.

In the preferred configuration, a translation surface or mountingsurface holds the contact points and the wafer or specimen is fastenedto a translation stage, which provides translation or sliding of thespecimen within and, into the lensing/imaging arrangement. The systemfirst performs an inspection of one portion of the specimen, and thetranslation stage and wafer are repositioned or translated such as bydriving the translating stage so that another portion of the wafer iswithin the imaging path. The other portion of the wafer is then imaged,and both two sided images of the wafer are “stitched” together.Optionally, more than two scans may be performed and stitched together.The number of scans relates to the size of the wafer and the collimatorsand cameras used. Smaller components tend to be less costly, and thuswhile performing more than one scan may introduce stitching errors andrequire additional time to perform a scan of the entire surface, such asystem may be significantly less expensive.

Other means for presenting the remaining portion of wafer or specimenmay be employed, such as rotating the wafer mechanically or manually, orkeeping the wafer fixed and moving the optics and imaging components.Alternately, scanning may be performed using multiple two-sidedinspections of the module, such as three, four, or five or more scans ofapproximate thirds, quarters, or fifths, and so forth of the specimen.While multiple scans require additional time and thus suffer fromincreased throughput, such an implementation could provide for use ofsmaller optics, thereby saving overall system costs.

In a two phase scan of a dual sided specimen, at least 50 percent of thesurface must be scanned in each phase of the scan. It is actuallypreferred to scan more than 50 percent, such as 55 percent, in each scanto provide for a comparison between scans and the ability to “stitch”the two scans together.

Scanning and stitching involves determining the piston and tilt of thespecimen during each scan, adjusting each scan for the piston and tiltof said scan, and possibly performing an additional stitching procedure.Additional stitching procedures include, but are not limited to, curvefitting the points between the overlapping portions of the two scansusing a curve fitting process, replacing overlapping pixels with theaverage of both data sets, or weighting the averaging in the overlappingregion to remove edge transitions by using a trapezoidal function, halfcosine function, or other similar mathematical function. Backgroundreferences are preferably subtracted to improve the stitching result. Ifsignificant matching between the scans is unnecessary, such as in thecase of investigating for relatively large defects, simply correctingfor tilt and piston may provide an acceptable result. However, in mostcircumstances, some type of curve fitting or scan matching is preferred,if not entirely necessary.

These and other objects and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddescription of the invention and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates the general concept of the predecessor Muellersystem for inspecting both sides of a semiconductor wafer or specimenwhen said specimen is oriented in a substantially “vertical”orientation;

FIG. 1B is a preferred embodiment of the current invention;

FIG. 1C illustrates a holding arrangement for use in the currentinvention, including a damping bar and dual sided lensing arrangement;

FIG. 1D is a conceptual illustration of the anamorphic imaging systemused in the system disclosed herein;

FIG. 1E shows a single channel camera system setup employed in thecurrent system, including the optical components between the specimen orwafer and the CCD;

FIG. 2 presents the operation of mounting points for the wafer orspecimen;

FIG. 3 illustrates a measurement module for use in connection withtranslating the wafer and performing multiple scans in the presence ofmultiple damping bars;

FIG. 4A shows the first position of the wafer or specimen relative to adamping bar when a rotational scanning and stitching procedure isperformed on approximately half the wafer surface;

FIG. 4B is the second, position of the wafer or specimen relative to adamping bar when a rotational scanning and stitching procedure isperformed on the other approximately half of the wafer surface;

FIG. 5A shows the first position of the wafer or specimen relative to adamping bar arrangement when a translational scanning and stitchingprocedure is performed on approximately half the wafer surface;

FIG. 5B is the second position of the wafer or specimen relative to adamping bar arrangement when a translational scanning and stitchingprocedure is performed on the other approximately half of the wafersurface;

FIG. 6 represents an algorithm for performing the scanning and stitchingaccording to the present invention;

FIG. 7 presents a conceptual schematic representation of the componentsand optics necessary to perform the inventive dual sided imaging of asemiconductor wafer; and

FIG. 8 is a top view of the components and optics showing the path oflight energy.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1B illustrates the reduced coherence inspection device of thecurrent invention. According to FIG. 1B, a variable coherence lightsource 151 is employed. The variable coherence light source 151 may be,for example, a helium-neon laser, but generally any type of variablecoherence light providing sufficient illumination characteristics forthe apparatus and method described herein is acceptable. The variablecoherence light source transmits light energy to collimator orcollimating lens 152, which directs the light to first diffractiongrating 153, The collimator 152 divides the light energy into twoseparate channels. First diffraction grating 153 widens the nth order(n>0) wave fronts of the light energy and directs the widened lightenergy toward the specimen being examined. As shown in FIG. 1B, lightenergy is directed toward the specimen and a pair of flat reflectingsurfaces 154 and 155, where the flat reflecting surfaces may have eitheropaque characteristics, such as a standard mirror surface, or be semitransparent, i.e. transparent through one side and <90% reflective onthe other. The dotted lines representing the waveform illustrated inFIG. 1 represent the higher order, such as first order, components ofthe light energy passing through the diffraction grating 153. The use ofa zero order blocking surface (not shown) may be included in the systemto prevent passage of the zero order component of the light energyemanating from the diffraction grating 153. The blocking surface may beany type of opaque surface, such as a light absorbing surface,dimensioned to prevent passage of zero order light components and permitthose higher order components illustrated in FIG. 1B to reflect in themanner illustrated. Alternatively, a diffraction grating optimized forzero intensity of its zero order can be employed, negating the need formechanical screens.

Light energy from each of the two channels strikes the specimen 111 andeach channel further reflects off a respective flat 154 or 155. Light isthereby deflected toward the second diffraction grating 156, whichcombines the reflected energy received from the flat 154 or 155 and thespecimen surface. Second diffraction grating 156 receives and combinesthe two channels of information and passes each channel of opticalinformation through a collimator to a camera. Collimators 157 and 158decollimate the light energy received from the second diffractiongrating 156.

In the system illustrated in FIG. 1B, reference surfaces and specimensurfaces are positioned such that the reference wave fronts and specimenwave fronts travel the same path length. Phase shifting may beestablished by moving the reference surfaces, the diffraction gratings,or the light source. Thus the overall effect of the system illustratedin FIG. 1B is to decrease the spatial coherence between the referencewave fronts and the specimen wave fronts.

As shown in FIG. 1B, an optional interferometric normal incidenceinspection device may be employed in the system described above,including a light emitting device, such as a laser 171, a beam splitter172, and a collimator 173. The flat 155 serves as a reference surface.Light emitted from the light emitting device passes through the beamsplitter toward the collimator, which collimates the light beam andpasses it through the, in this case, semi transparent, flat 155 andtoward the specimen 111. Light then reflects from the surface of thespecimen 111 and from the reflective surface of the flat 155 facing thespecimen, through the flat 155, through the collimator 173, and towardthe beam splitter 172. The beam splitter 172 directs the reflected beamto supplemental collimator 174 and to a camera arrangement. Thisapparatus provides for an additional channel of inspection and can becombined with the illustrated two channels of information to provideadditional information enhancing the quality of the scan. Further, thenormal incidence arrangement may also be employed on the other side ofspecimen 111, providing yet further optical information of the specimensurface.

The camera system 159 comprises camera arrangement 159A and cameraarrangement 159B. The camera system or anamorphic imaging system has anaspect ratio of on the order of 2:1. In essence, the wafer in theconfiguration illustrated optically appears as a tilted object, and inthe arrangement shown has an elliptical projection ratio ofapproximately 6:1. The camera system used should preferably resolve thiselliptical projection ratio into an image having an aspect ratio closerto 1:1. Maintaining the aspect ratio of 6:1 can prevent detection ofrelatively significant magnitude.

The overall configuration of the anamorphic imaging system used in thesystem disclosed herein is shown in FIG. 1D. From FIG. 1D, theprojection of the image has, an elliptical aspect ratio of 6:1. Theanamorphic imaging system 166 receives the elliptical image 167 andconveys the image to a viewing location, such as a CCD (Charged CoupledDevice) such that, the received image 168 has an aspect ratio of 2:1.This ratio provides the maximum utilization of a square image whenimaging each of the wafer stitching regions. Different anamorphicimaging arrangements may be employed while still within the scope of thecurrent invention; the intention of the an amorphic system and functionthereof is to provide a sufficient image based on the surfaces beingscanned and the size and quality of defects expected, as well as theresolution capability of the overall system.

A simplified drawing of the system from the wafer to the cameraarrangement 153 is presented in FIG. 1E. FIG. 1E is not to scale andrepresents a single channel of inspection rather than a dual channel anddual camera arrangement. From FIG. 1E, wafer or specimen 111 reflectsthe light energy toward second diffraction grating 156, which, passeslight to collimator 173, comprising decollimating lenses 173A and 173B,and to a camera arrangement 159A. Camera arrangement 159A comprisesseven imaging lenses used to resolve the 6:1 image received into a 2:1image for transmission to CCD 160. Any lensing, arrangement capable ofproducing this function is acceptable, and the camera arrangement 159Ais therefore not limited to that illustrated in FIG. 1E.

An additional feature of the current system is the use of a calibrationobject for, distortion calibration. In the system illustrated, acalibration object 175 (not shown) is used in place of wafer or specimen111. The use of a calibration object provides a known reference whichenables accurate matching of images on the front and back side of thewafer 111 with sub pixel accuracy. The use of the calibration object 175permits calculation of the thickness variation of the specimen bydetermining the difference between the front and back topography maps ofthe specimen. The calibration object is similar to a wafer having thesame pattern on the front and backsides at the same coordinates. Theaccuracy of the calibration object features is detectable using thesystem/interferometer with pixel accuracy. One type of calibrationobject employs a symmetric pattern of circular raised features havingrelatively small diameters/pitches, such as on the order of 5 to 10millimeters, and covering both the front and back surfaces. Otherpatterns, pitches, and spacings may be employed as long as the precisionof the measuring device may be determined.

In operation, the calibration object 175 is placed as shown in FIGS. 1Band 1C and images of the front surface and back surface obtained. Thefeatures on, the front and back surfaces of the calibration object aremeasured and their locations are determined to within the desiredaccuracy. A spatial transformation is computed which maps the measuredlocations of the features to their actual locations. The wafer orspecimen 111 is positioned in the system as shown, with sufficient caretaken to place the wafer or specimen 111 in an identical position to thecalibration object 175. The specimen is then examined on both front andback sides and the thickness variation determined by applying the samespatial transformation as for the calibration wafer

FIG. 1C illustrates scanning both sides of a dual-sided wafer orspecimen 111. According to FIG. 1C, the wafer 111 is mounted using afixed three point mounting arrangement as shown in FIG. 2. The, threepoint mounting arrangement serves to hold the wafer 111 at a relativelyfixed position while simultaneously minimizing any bending or stressingof the dual-sided wafer. Light energy is transmitted through firstcollimating lens 112 arranged to cause light energy to strike thesurface of the wafer 111 and subsequently pass through secondcollimating lens 113 where detection and observation is performed. Asmay be appreciated by examining FIG. 1C, the diameter of both firstcollimating lens 112 and second collimating lens 113 are significantlysmaller than the diameter of the specimen or wafer 111, and incidentlight strikes only a portion of the surface of wafer 111. Not shown inthe illustration of FIG. 1C is that while light energy is striking thesurface of wafer 111 visible in the arrangement shown, light energysimultaneously passes through first collimating lens 112 and strikes thereverse side of the wafer 111, not shown in FIG. 1C. Light energy passesfrom the reverse side of the specimen 111 through second collimatinglens 113.

The arrangement further includes an upper damping bar 114 and a lowerdamping bar 115. In the arrangement shown in FIG. 1C, the upper dampingbar 114 covers approximately one half of the specimen 111, specificallythe half not being inspected. The effect of the damping bar is to dampthe non-measured surface of the specimen 111 to minimize the effects ofvibration. Damping in this arrangement is based on VFD, or the Bernoulliprinciple, wherein the upper damping bar 114 in the arrangement shown isbrought to within close proximity of the surface to be damped. Theproximity between either damping bar 114 or 115 and the surface of thewafer is preferably less than 0.5 millimeters, and spacing of 0.25 and0.33 may be successfully employed. The problem associated with providingsmaller gaps between either damping bar 114 or 115 and the surface ofwafer 111 is that any warping of the wafer may cause the bar to contactthe surface. For this reason, and depending on the wafer surface, gapsless than 0.10 millimeters are generally undesirable. Further, gapsgreater than 1.0 millimeters do not produce a desirable damping effect,as the Bernoulli principle does not result in sufficient damping in thepresence of gaps in excess of 1.0 millimeter.

The gap between the specimen 111 and upper damping bar 114 or lowerdamping bar 115 restricts airflow between the specimen and the dampingbar and damps vibration induced in the specimen. Each damping bar isgenerally constructed of a stiff and heavy material, such as a solidsteel member. Overall dimensions are important but not critical in thatthe damping bar should cover a not insignificant portion of the wafer111. Coverage of less than 20 percent of the wafer tends to minimize theoverall damping effect on the wafer, but does provide some level ofdamping.

The illumination of only a portion of the wafer 111 permits use ofsmaller lenses than previously known. In the embodiment shown in FIG.1B, the preferred size of the first collimating lens 112 and secondcollimating lens 113 is approximately 4.4 inches where the wafer 111 is300 millimeters in diameter. In such an arrangement, the damping bars114 and 115 are approximately 4.5 inches wide. Length of the dampingbars depends on the mode of wafer movement, as discussed below.

As shown in FIG. 2, the mounting for the wafer 111 is preferably using athree point kinematic mount, where the three points 201, 202, and 203represent spherical or semispherical contacts tangential to one another.Points 201, 202, and 203 are small clips having spherical orsemi-spherical tangentially mounted contacts, mounted to a support platesuch as mounting plate 116 to be substantially coplanar, with adjustableclips to provide for slight irregularities in the shape of the wafer111. The spherical or semispherical components should be sufficientlyrigid but not excessively so, and a preferred material for thesecomponents is ruby. The adjustability of points 201, 202, and 203provide an ability to hold the wafer 111 without a stiff or hardconnection, which could cause bending or deformation, as well as withouta loose or insecure connection, which could cause inaccuratemeasurements. In FIG. 1C, two lower kinematic mount points 202 and 203(not shown) support the lower portion of the wafer 111, while the upperportion is supported by mount point and clip 201. The points 201, 202and 203 are therefore stiff enough to mount the wafer or specimen 111and prevent “rattling” but not so stiff as to distort the wafer. Thespherical or semispherical contact points are generally known to thoseof skill in the mechanical arts, particularly those familiar withmounting and retaining semiconductor wafers. The combination of clampingin this manner with the Bernoulli damping performed by the damping bars114 and 115 serves to minimize acoustic and seismic vibration.

Simultaneous imaging of both sides of the specimen is generallyperformed in accordance with PCT Application PCT/EP/03881 to DieterMueller, currently assigned to the KLA-Tencor Corporation, the assigneeof the current application. The entirety of PCT/EP/03881 is incorporatedherein by reference. This imaging arrangement is illustrated in FIGS. 7and 8, and is employed in conjunction with the arrangement illustratedand described with respect to FIG. 1B herein. FIGS. 7 and 8, as well asFIG. 1B, are not to scale. As shown in FIGS. 7 and 8, the light energydirecting apparatus employed in the current invention comprises a lightsource in the form of a laser 801. The light emitted from the laser 801is conducted through a beam waveguide 802. The light produced by thelaser 801 emerges at an end 803 of the beam waveguides 802 so that theend 803 acts as a punctual light source. The emerging light strikes adeviation mirror 804 wherefrom it is redirected onto a collimationmirror 807 in the form of a parabolic mirror by two further deviationmirrors 805 and 806. Deviation mirrors 805 and 806 are oriented at anangle of 90° relative to each other. The parallel light beam P reflectedfrom the parabolic mirror 807 reaches a beam splitter 808 through thetwo deviation mirrors 805 and 806.

The beam splitter 808 is formed as a first diffraction grating. The beamsplitter 808 is arranged in the apparatus in a vertical direction andthe parallel light beam P strikes the diffraction grating in aperpendicular direction. A beam collector 810 in the form of a seconddiffraction grating is disposed from the first diffraction grating 808and parallel thereto. Behind the beam collector 810 two decollimationlenses 811 are arranged at equal level and the light beams leaving thesedecollimation lenses are each deflected and focused onto two CCD cameras816A and 816B, through deviation mirror pairs 812A and 812B, 813A and813B, and 814A, and 814B, and to an optical imaging system 15.

The beam splitter 808 is supported transversely to the optical axis andfurther comprises a piezoelectric actuating element 817 for shifting thephase of the parallel light beam P by displacing the diffractiongrating.

A holding device 830, for example the holding device disclosed hereinand described with respect to FIGS. 1C, 2, and 3, is provided betweenthe first diffraction grating and the second diffraction grating. Otherholding devices may be employed while still within the scope of thisinvention, such as a support post. A wafer or specimen 809 to bemeasured is held on the holding device 830 such that both plane surfaces831 and 832 are arranged in vertical direction parallel to the lightbeam P. The wafer 809 is supported by the support post substantially atits vertical edge 833 only so that both surfaces 831 and 832 are notsubstantially contacted by the support post and are freely accessible tothe interferometric measurement.

Moreover, an optional receiving device (830, 825) may be provided formeasuring the wafer 809. This receiving device (830, 825) provides forarrangement of the wafer in the system and provides an alternative tothe wafer maintaining device shown in FIGS. 1C, 3, and 4. The wafer canbe inserted into the receiving device in a horizontal position. By meansof a tilting device 826 the wafer 809 may be tilted from its horizontalposition into the vertical measuring position, and the wafer 809 may betransferred, by means of a positionable traveller, into the light pathbetween the first diffraction grating and the second diffraction gratingso that the surfaces 809 and 832 to be measured are alignedsubstantially parallel to the undiffracted light beam P and in asubstantially vertical direction.

Furthermore, a reference apparatus 820 may be provided which comprises areference body 821 having at least one plane surface 824. The referencebody 821 can be introduced into the light path between the firstdiffraction grating 808 and the second diffraction grating 810 in placeof the semiconductor wafer or specimen 809 to be measured by means of atraveller 823 with a linear guide 818. The reference body 821 is held sothat its plane surface 824 is arranged in vertical direction parallel tothe undiffracted light beam P. The reference body 821 can be turned by180° in its mounting around an axis parallel to its surface 824.

In operation the wafer or specimen 809 to be measured is first insertedinto the wafer receiving device 825. The surfaces 831 and 832 arehorizontally arranged. By means of the tilting device and of thetraveller 819 the wafer to be measured is brought into the holdingdevice 830 where it is arranged so that the surfaces 831 and 832 arevertical. A diffraction of the parallel light beam P striking the firstdiffraction grating 808 of the beam splitter produces partial lightbeams A, B, whereby the first order component of the partial light beamA having a positive diffraction angle strikes the one surface 831 of thewafer 809 and is reflected thereat. The first order component of partiallight beam B with a negative diffraction angle strikes the other surface832 of the wafer and is reflected thereat. The first order component ofpartial light beams A and B each strike the respective flat, or mirroredsurface, where the first order component of partial light beam A strikesflat 851, and first order component of partial light beam B strikes flat852. The 0-th diffraction order of the parallel light beam P passesthrough the first diffraction grating 808 and is not reflected at thesurfaces 831 and 832 of the wafer 809. This partial light beam P servesas references beam for interference with the reflected wave fronts ofthe beams A and B. Each 0-th order beam is preferably blocked byblocking surfaces 853 and 854. In the second diffraction grating 810,the beam collector and the reflected first order components of partiallight beams A and B are each combined again with the reference beam Pand focused, in the form of two partial light beams A+P and B+P onto thefocal planes of the CCD cameras 816A and 816B through decollimationlenses 811 and deviation mirrors 812, 813 and 814 as well as positivelenses 815.

During the exposure of the surfaces the phase of the parallel light beamP is repeatedly shifted by multiples of 90° and 120° by displacing thediffraction grating. This produces phase shifted interference patterns.The defined shift of the interference phase produced by the phaseshifter 817 is evaluated to determine whether there is a protuberance ora depression in the measured surfaces 831 and 832 the two digitizedphase patterns are subtracted from each other.

A calibration using the reference body 821 may optionally be performedbefore each measurement of a wafer 809. The reference body 821 isintroduced into the beam path between the first diffraction grating 808and the second diffraction grating 810. The known plane surface 824 ismeasured. Subsequently the reference body 821 is turned by 180° and thesame surface 824 is measured as a second surface.

FIG. 3 illustrates the measurement model without a wafer or specimenpresent. From FIG. 3, light source 301 initially emits light energy andis focused to strike first mirror surface 302 and second mirror surface303 (not shown). Each of these two mirror surfaces direct light energythrough first collimating lens 112 (not shown in this view) and lightenergy strikes the two surfaces of specimen 111 (also not shown)simultaneously. After striking the two surfaces of specimen 111, lightenergy is directed through second collimating lens 113 (also not shownin FIG. 2) and to third mirror 304 and fourth mirror 305, which directlight energy toward focusing element 306 and detector 307. Imaging arm311 represents the light image path from third mirror 304 towardfocusing element 306. Focusing elements and sensors are those known inthe art, and may include a lensing arrangement, such as multiple lenses,and a CCD or other imaging sensor. Other implementations of focusingelement 306 and detector or sensor 307 are possible while still withinthe scope of the current invention.

From FIG. 1C, the specimen 111 is mounted to three points, includingpoint 201, which are fixedly mounted to mounting surface 116. Mountingsurface 116 may be fixedly mounted to translation surface 117. Eithertranslation surface 117 or mounting surface 116 is fastened totranslation stage 308, which provides translation or sliding of themounting surface 116 and specimen 111 within and into the arrangementshown in FIG. 3. The arrangement may further include translation surface117 depending on the application. Translation stage 308 permits thearrangement of FIG. 1B, specifically wafer or specimen 111, points 201,202, and 203, mounting surface 116, and translation surface 117, to moveup and down in a relatively limited range, as described below. In suchan arrangement employing translation surface 117, the translationsurface and the mounting surface along with the contact points arepositioned within the measurement module 300, preferably by affixing thetranslation surface 117 to the translation stage 308. Specimen 111 isthen physically located between damping bars 114 and 115, as well asproximate damping bar 309 and fastened to points 201, 202, and 203. Oncethe specimen 111 has been adequately fastened to points 201, 202, and203, an inspection of the lower portion of the wafer is initiated. Aftercompleting an adequate inspection, i.e. an inspection of one portion ofthe specimen 111 with acceptable results, the translation stage 308 andultimately the wafer are repositioned or translated such as by drivingthe translating stage 308 along track 310 such that another portion ofthe wafer 111, such as the remaining approximately half of specimen 111is within the imaging path. The other portion of the wafer is thenimaged, and both of the two sided images of the wafer surface are“stitched” together.

The damping bars may have varying size while still within the scope ofthe current invention, as discussed above. In FIG. 3, the damping barsare affixed to end pieces 310 and 311, but any type of mounting willsuffice as long as the gap spacing described above and the ability toperform scans on desired portions of the wafer is available.

As may be appreciated, other means for presenting the remaining portionof wafer or specimen 111 may be employed, such as rotating the wafer byhand by releasing contact with the points and rotating the wafermanually. Alternately, a mechanical rotation of the specimen may occur,such as by rotatably mounting the mounting surface 116 on thetranslating surface 117 while providing for two locking positions forthe mounting surface 116. In other words, the arrangement of wafer 111,points 201, 202, and 203, and mounting surface 116 would initiallyfixedly engage translation surface 117. On completion of a firstinspection scan of a portion of specimen 111, wafer 111, points 201,202, and 203, and mounting surface 116 would be unlocked fromtranslation surface 117 and be mechanically or manually rotatedvertically on an axis perpendicular to translation surface 117. Thewafer and associated hardware rotate 180 degrees to a second lockingposition, wherein the surface would lock and a second inspection scanwould commence. During this rotation scheme, damping bars andimpediments would be mechanically or manually removed to prevent contactwith mounting points 201, 202, and 203. The various components,particularly mounting surface 116, are sized to accommodate rotationwithin the measurement module 300 without contacting the translationstage or other module components.

Alternately, scanning may be performed using multiple two-sidedinspections of the module, such as three, four, or five scans ofapproximate thirds, quarters, or fifths of the specimen. While multiplescans require additional time and thus suffer from increased throughput,such an implementation could provide for use of smaller optics, therebysaving on system costs. Numerous sub-aperture scans may be performed bya system similar to that illustrated in FIG. 3 while still within thescope of the current invention.

FIGS. 4A and 4B illustrate a rotational scanning arrangement of thewafer or specimen 111. As may be appreciated, in a two phase scan of adual sided specimen, at least 50 percent of the surface must be scannedin each phase of the scan. It is actually preferred to scan more than 50percent, such as 55 percent, in each scan to provide for a comparisonbetween scans and the ability to “stitch” the two scans together. Insuch an arrangement, as shown in FIG. 4A, over 50 percent of the surfaceis scanned initially, shown as portion A of the surface 111. Portion Bis obscured by one of the damping bars. After the initial scan phase,the specimen 111 is rotated manually or mechanically to the positionillustrated in FIG. 4B. Approximately 55 percent of the wafer surface,both front and back, are scanned during this second phase. This providesan overlap of five percent of the wafer, and comparisons between theseoverlap portions provides a reference for stitching the scans together.In FIG. 4B, the A portion of the wafer is obscured by the damping bar.

Alternately, as in the arrangement shown in FIG. 3, the wafer orspecimen 111 may be translated vertically and two or more separate scansperformed. As shown in FIGS. 5A and 5B, a portion of the wafer 111 ispositioned between two damping, bars, such as damping bars 114 and 115,and the portion marked “B” in FIG. 5A is scanned. As shown therein,greater than 50 percent of the specimen 111 is scanned so that theoverlapping portion may be stitched with the second scan. After theinitial scan, the wafer is translated to a position as shown in FIG. 5B.Portion “A” of FIG. 5B is then, scanned, while the lower damping barcovers much of section “B.” The overlapping portions of the two scansare then stitched together to provide a full representation of thesurface, and again such a scan is dual-sided.

From FIGS. 4A, 4B, 5A, and 5B, it should be apparent that a singledamping bar is required if the specimen 111 is to be rotated as shown inFIGS. 4A and 4B, while two damping bars are required if the wafer 111 isto be translated, as shown in FIGS. 5A and 5B. Note that due tomeasurement setup, an arbitrary piston or DC offset and tilt will beapplied to each of the measurements, indicating that some correction isrequired prior to or during stitching to obtain an accurate surfacerepresentation.

FIG. 6 illustrates a general scanning and stitching algorithm for use inaccordance with the invention described herein. The algorithm begins instep 601 and performs the first scan in step 602, as well as determiningthe piston and tilt of the specimen 111. The algorithm evaluates whetherthe scan is acceptable in step 603, either performed by an operatoractually evaluating the scan or a mechanical comparison with a known orprevious scan. If the scan is acceptable, the algorithm proceeds to step604 where the wafer is repositioned to the next location. If the scan isnot acceptable, the wafer is rescanned in its original position. Pistonand tilt may be recomputed, but as the wafer has not moved this is notnecessary. Once the wafer has been repositioned in step 604, asubsequent scan is performed in step 605 and the tilt and pistoncomputed for the new orientation. The acceptability of the scan isevaluated in step 606, and if unacceptable, the scan performed again.The piston and tilt again do not need to be recalculated. Once the scanis mechanically or visually deemed acceptable, the algorithm determineswhether the entire surface has been scanned in step 607. If the entiresurface has not been scanned, the wafer is again repositioned and theremaining scans performed in accordance with the illustrated steps. Ifthe entire surface has been scanned, the algorithm sets x equal to oneand y equal to 2 in step 608. In step 609 the system alters scan x fortilt and piston and separately alters scan y for its respective tilt andpiston. At this point scans x and y are neutrally positioned and may bestitched together. Step 610 is an optional step of performing anadditional stitching procedure. Additional stitching procedures include,but are not limited to, curve fitting the points between the overlappingportions of the two scans using a curve fitting process, replacingoverlapping pixels with the average of both data sets, or weighting theaveraging in the overlapping region to remove edge transitions by usinga trapezoidal function, half cosine function, or other similarmathematical function. Background references are preferably subtractedto improve the stitching result. If significant matching between thescans is unnecessary, such as in the case of investigating forrelatively large defects, simply correcting for tilt and piston mayprovide an acceptable result, and step, 610 need not be performed.However, in most circumstances, some type of curve fitting or scanmatching is necessary. Scans are matched and stitched in step 611. Suchstitching algorithms should preferably be performed using a computingdevice, such as a microprocessor (not shown).

Step 612 evaluates whether the complete wafer has been stitchedtogether. If it has not, the algorithm proceeds to increment x and y instep 613 and perform additional stitching, of the remaining portions. Ifthe complete wafer has been stitched, the algorithm exits in step 614.

Based on the disclosure presented above and in particular in connectionwith that shown in FIG. 3, the wafer 111 is generally repositioned whilethe inspection energy source and optics remain fixed. While thisimplementation provides distinct advantages in setup time for performingmultiple dual-sided wafer scans, it is to be understood that the lightsource and associated optics and detector may be slidably orrotationally mounted while the wafer remains fixed. In the configurationillustrated in FIG. 3, source 301, support elements 310 and 311, dampingbars 114 and 115, damping bar 309, the four mirrors 302, 303, 304, and305, focusing element 306, and detector 307 may be mounted to a singlesurface and fixedly positioned relative to one another, and translatedor rotated about the wafer. Alternately, the components may betranslated either together or individually to perform subsequent scansof the wafer or specimen 111.

While the invention has been described in connection with specificembodiments thereof, it will be understood that the invention is capableof further modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as come within known and customary practice withinthe art to which the invention pertains.

What is claimed is:
 1. A method for measuring specimen thicknessvariations in a specimen having at least two sides, the methodcomprising: transmitting variable coherence light energy; receiving thevariable coherence light energy, forming diffracted light energy, andpassing the diffracted light energy toward two sides of said specimenand toward a plurality of reflective surfaces spaced apart from saidspecimen, the plurality of reflective surfaces comprising asemi-transparent reflective surface transparent through a first side andless than 90 percent reflective on a second side; sensing light energyfrom the specimen and the plurality of reflective surfaces; andperforming an interferometric normal incidence inspection through thesemi-transparent reflective surface.
 2. The method of claim 1, whereinforming diffracted light energy comprises diffracting coherence lightenergy such that the passing the diffracted light energy passesdiffracted nonzero order light energy.
 3. The method of claim 2, furthercomprising blocking passage of diffracted zero order light energy. 4.The method of claim 1, further comprising receiving light energy fromsaid plurality of reflective surfaces at a diffraction grating beforesaid sensing.
 5. The method of claim 1, wherein the sensing comprisesreceiving light energy from the plurality of reflective surfaces and thespecimen at a sensor.
 6. The method of claim 1, wherein the sensingemploys a sensor comprising at least one camera, wherein each cameraconverts an elliptical image of at least one side of said specimen intoan image having an aspect ratio closer to 1:1.
 7. The method of claim 4,further comprising collimating variable coherence light energy andfurther collimating light energy received from said diffraction grating.8. The method of claim 1, wherein said receiving comprises: passingnonzero order light energy toward at least one reflective surface andsaid specimen; and simultaneously blocking zero order light energy. 9.The method of claim 1, wherein the interferometric normal incidenceinspection employs a collimator.
 10. The method of claim 9, wherein saidinterferometric normal incidence inspection comprises emitting lighttoward a beamsplitter and the collimator.
 11. The method of claim 1,further comprising optimizing diffracting for zero intensity of a zeroorder.
 12. A method for measuring thickness variations of a specimencomprising a plurality of sides, the method comprising: diffractingvariable coherence light energy into multiple channels of light energy;directing said multiple channels of light energy toward two sides ofsaid specimen and toward multiple reflecting surfaces, the multiplereflecting surfaces comprising a semi-transparent reflecting surfacetransparent through a first side and less than 90 per cent reflective ona second side; sensing light energy received from the two sides of thespecimen; and performing an interferometric normal incidence inspectionof the specimen through the semi-transparent reflecting surface.
 13. Themethod of claim 12, wherein said diffracting comprises diffracting forzero intensity of the zero order of the multiple channels of lightenergy received.
 14. The method of claim 12, further comprisingperforming an initial calibration.
 15. The method of claim 12, whereinperforming the interferometric normal incidence inspection on thespecimen occurs prior to said diffracting.
 16. The method of claim 12,wherein performing the interferometric normal incidence inspection ofthe specimen occurs after said sensing.
 17. The method of claim 12,wherein said light energy received from the two sides of the specimenforms an image, and said directing step comprises altering the imageaspect ratio.
 18. A specimen thickness measurement method, comprising:diffracting variable coherence light energy toward a specimen having aplurality of sides; receiving light energy from said diffracting at aplurality of reflecting surfaces and at two sides of said specimen, theplurality of reflecting surfaces comprising a semi-transparentreflecting surface transparent through a first side and less than 90percent reflective on a second side; and performing an interferometricnormal incidence inspection of the specimen through the semi-transparentsurface; wherein said diffracting directs energy simultaneously towardone reflecting surface and one side of the specimen.
 19. The method ofclaim 18, further comprising sensing light energy received from theplurality of reflecting surfaces and said specimen to determinethickness variations in said specimen.
 20. The method of claim 18,further comprising blocking zero order light energy received from saiddiffracting.